The present invention relates generally to lasers and more particularly to laser cavities.
Over the last twenty years the use of lasers in science, health care and industry has received wide acceptance in an ever increasing variety of applications. Lasers have found use in such diverse areas as range finding apparatus, optical surgery, optical printers, optical readers and metal drilling. As is known, lasers operate on the principle of light amplification through stimulated emission of radiation and can create extremely intense concentrations of light. Briefly, a laser includes a laser material, sometimes referred to as an active element, a mechanism for pumping the laser material and an optical resonant cavity, sometimes referred to simply as a laser cavity. The laser material is disposed in the laser cavity and is pumped by the pumping mechanism to emit light. Light emitted by the laser material is reflected back and forth within the cavity from one end to the other, with a portion of the light being transmitted out of the cavity for subsequent use.
Materials which are used as laser materials include gases, liquids, glasses and single crystalline solids. The mechanism which is employed for pumping the laser material is either electrical or optical, the particular mechanism used depending on the particular laser material. The laser cavity usually comprises a pair of end mirrors which are spaced from one another at an appropriate distance and angularly arranged to define a closed loop optically resonant cavity. The end mirrors may both be flat, one may be curved or both may be curved. At least one of the end mirrors is made partially transmissive to allow a portion of the radiation emitted from the laser material and reflected back and forth between the end mirrors to leave the cavity.
In some lasers the output is in the form of a continuous wave while in other lasers the output is in the form of a train of pulses. Generally, the distance that light travels in the optical cavity when going from one end mirror to the other end mirror through the laser material is called the optical path length of the cavity. The time it takes for light traveling inside the cavity to go from one end mirror to the other end mirror and then back to the first end mirror, is commonly referred to as the "round trip cavity time." In a pulse laser, the spacing between the individual pulses in the chain of pulses that are emitted by the laser is dependent on the optical path length of the cavity and, correspondingly, the round trip cavity time. The larger the optical path length, the greater the separation between pulses. During the past ten years, ultrafast (i.e. picosecond) pulses have been generated in the visible and infrared spectral regions by passive and active mode-locked ruby, neodymium and dye laser and other types of laser systems.
Examples of some types of lasers may be found in U.S. Pat. No. 4,464,741 to R. Alfano et al, U.S. Pat. No. 4,272,733 to J. C. Walling etc., U.S. Pat. No. 3,997,853 to R. C. Morris, and U.S. Pat. No. 3,508,166 to W. W. Simmons et al.
One laser that has proven to be the workhorse for investigating picosecond and nonlinear optical phenomena is the solid state laser. An example of a solid state laser is the mode-locked neodymium glass laser. The output of a conventional mode-locked solid state laser consists of a train of approximately 100 pulses emitted over a time period of about 500 nanoseconds (ns) with each pulse separated by about 5 to 10 ns. The pulse widths are on the order of 6 to 10 picoseconds (ps) with a peak power of 2 to 5 GW. The optical path length of the laser cavity in such a laser usually ranges from approximately 1 to 2 meters, due to limited space and mechanical stability.
In a number of situations, such as ranging and remote sensing, it has been desirable if not essential to be able to have a pulse to pulse spacing that is much larger than the spacing that is actually produced by the laser itself.
One way that has been used in the past to increase the space between pulses has involved increasing the spacing between the end mirrors which make up the laser cavity i.e. moving one end mirror further away from the other. As can be appreciated, increasing the geometric distance between the cavity end mirrors does increase the optical path length of the cavity and will result in an increase in the spacing between the output pulses; however, this involves physically moving one or both end mirrors and results in actually reconstructing the entire cavity. Furthermore, the amount by which the two end mirrors must be spaced from one another to provide a particular pulse to pulse spacing is in some cases just not practical.
Another way that has been employed in the past to increase the pulse to pulse spacing of the pulses emitted from a pulse laser has involved not making any changes in the laser itself, but rather blocking out certain pulses in the output chain produced by the laser and not blocking out other pulses. For example, three out of every four pulses in the train may be blocked out and the fourth pulse allowed to pass. This has been achieved using either a chopper, an acousto-optical modulator, an electro-optical shutter or other type of similarly functioning device, with the device being located outside of the cavity. The problems with this approach are that it is rather complicated, not very practical and not entirely satisfactory in all situations.
As can be appreciated, the need exists for a new and improved technique for simply, effectively and efficiently increasing the optical path length of a laser cavity.